Cascade refrigeration systems are conventionally used to regulate the temperature in test chambers which form a part of environmental testing apparatus. In such cascade refrigeration systems, evaporator coils mounted in the test chamber are provided with a cooled refrigerant so as to remove heat from the chamber and thus lower the chamber temperature. The cooled refrigerant is supplied to the evaporator coils by a conventional compressor-condenser system. The refrigerant used in this system is continuously cycled in a vapor-compression refrigeration cycle.
In the cascade refrigeration system, a secondstage system including a compressor-condenser cools a refrigerant which is then passed in a heat-exchange relationship with the refrigerant of the first stage so as to lower the temperature of the first refrigerant before delivery to the evaporator coils of the chamber. This type of cascade refrigeration system is well known and conventional in environmental test chambers.
One such example of a test chamber including a cascade refrigeration system is shown in U.S. Pat. No. 3,590,595 issued July 6, 1971. The refrigeration system shown in the above-referenced patent includes a conventional cascade refrigeration system which is further equipped with a bypass system for removing refrigerant before it is delivered to the evaporator and delivering this refrigerant to the outlet side of the evaporator so as to provide for a liquid flow to the expansion valve at low-load conditions.
The apparatus shown in the '595 patent includes a bypass circuit in the form of a line which diverts refrigerant through a solenoid to an expansion valve. The expansion valve then supplies the cooled refrigerant to a bypass heat exchanger, which refrigerant is then supplied to the input of the first-stage compressor. The return of the heat exchanger is communicated to the outlet side of the evaporator. This bypass circuit is utilized during periods when the refrigeration requirements at the test chamber are reduced.
The above-described system is useful when piping refrigeration to remote units. However, this device does require the addition of a third pipe for bypassing refrigerant to the outlet side of the evaporator.